1. Field of the Invention
This invention in general relates to the fields of imaging and metrology of surfaces and, more specifically, to systems and methods for providing controlled phase evanescent field illumination for visualizing, imaging, energizing, and measuring submicron topographic features with enhanced lateral resolution.
2. Description of the Prior Art
The use of evanescent fields for measuring and visualizing submicron surface topographic features is known. Descriptions of evanescent field usage are contained, for example, in: Harrick, N.J., "Use of Frustrated Total Internal Reflection to Measure Film Thickness and Surface Reliefs," J. Appl. Phys., 1962. 33: p. 321; McCutchen, C. W., "Optical Systems for Observing Surface Topography by Frustrated Total Internal Reflection and by Interference," The Review of Scientific Instruments, Vol. 35, p. 1340-45, 1964; Guerra, J. M., September 1988, "Photon tunneling microscopy," in Proceedings from Surface Measurement and Characterization Meeting, Hamburg, SPIE Vol. 1009, pp. 254-62; U.S. Pat. No. 4,681,451, entitled "Optical proximity imaging method and apparatus," issued 21 Jul. 1987 to Guerra, J. M. and Plummer, W. T.; U.S. Pat. No. 5,349,443 entitled "Flexible transducers for photon tunneling microscopes and methods for making and using same," issued 20 Sep. 1994 to Guerra, J. M.; and, U.S. Pat. No. 5,442,443 entitled "Stereoscopic Photon Tunneling Microscope," issued 15 Aug. 1995 to Guerra, J. M., all patents assigned to Polaroid Corporation.
Harrick, McCutchen, Guerra/Plummer, and Guerra disclose whole-field reflected evanescent light microscopes where the sample is neither transilluminated nor scanned, but is rather illuminated by an evanescent field from an unrestricted total reflection surface at the object plane of an epi, or reflected-light, illuminator. Here, the sample can be opaque or transparent, thick or thin, and can be viewed in real-time with high energy throughput. Such microscopes are very sensitive to smooth surfaces because of their use of the exponentially varying amplitude of the evanescent field in the vertical direction to sense very small surface height variation. On the other hand, rougher surfaces scatter light back into the microscope, which decreases contrast and sensitivity. Also, the deeper topography is rendered as bright, because these areas penetrate the evanescent field to a small degree so that the epi-illumination is nearly totally reflected. The difficulty in detecting and measuring small changes in bright scenes limits the observable topographic depths to about 3/4 of the illuminating wavelength (which is the wavelength in air divided by the index n and the sine of the incident angle I). Further, the illumination and imaging optics are coupled because the objective element also serves as the condenser. In a practical sense, this limits the use of such instruments to the availability of suitable commercial objectives, magnifications, fields of view, and numerical aperture. In addition, it is difficult, because of the coupling of imaging and illumination optics, to affect the polarization, phase, incident angle, and direction of the illumination. This, in turn, restricts the ability to maximize the tunneling range, increase lateral resolution, or tunnel through less rare media such as water in biological applications.
Devices in which evanescent light from transilluminated samples is scattered into objective pupils are described in: G. J. Stoney, "Microscopic Vision," Phil. Mag. 332, at 348-9, 1896: Surface contact microscope, Taylor & Francis; Ambrose, E. J., "A Surface Contact Microscope for the Study of Cell Movements," Nature, Nov. 24, 1956, Vol. 178; Ambrose, E. J., "The Movements of Fibrocytes,"Experimental Cell Research, Suppl. 8, 54-73 (1961); Temple, P. A., "Total internal reflection microscopy: a surface inspection technique," Applied Optics, Vol. 20, No. 15 Aug. 1981; and, D. Axelrod, in Fluorescence Microscopy of Living Cells in Culture, Part B, ed. D. L. Taylor and Y-L. Wang, (Academic Press, New York, 1989), Chap. 9.
Stoney, Ambrose, Temple, and Axelrod disclose optical evanescent light field microscopes in which the light that enters the objective pupil is evanescent field light that has been scattered from a sample surface. However, in all of the microscopes described in the above references, the sample is transilluminated with the illumination incident at beyond the critical angle such that the evanescent field from the sample surface is received. In these applications, it is a requirement either that the sample material be transparent at optical frequencies or that the sample itself be thin enough to be transparent.
Scanning devices which rely on scattered evanescent field light are described in Fischer, U. Ch., Durig, U. T., and Pohl, D. W., "Near-field optical scanning microscopy in reflection," Appl. Phys. Lett., Vol. 52, No. 4, pp. 249-51, 25 Jan. 1988. Fischer et al. disclose a near-field optical microscope in which the sample is not transilluminated but is rather illuminated in reflected light. Further, this reflected light is in the form of an evanescent field from a dielectric plate into which light is launched at greater than the critical angle, by means of a coupled prism, so that it undergoes multiple total internal reflections, giving rise to the evanescent field. However, Fisher et al restrict the evanescent field with an aperture in a metal opaque coating on the total reflection surface of the dielectric plate. This aperture is smaller than the light wavelength so that an improvement in lateral resolution beyond the normal Abbe limit is achieved, but at the cost of having to scan the aperture relative to the sample to build up an image. A further cost is that energy throughput is very low, making extension to analytical optical techniques such as spectroscopy problematic.
Devices which utilize transillumination of transparent samples are described in R. C. Reddick, R. J. Warmack, and T. L. Ferrell, "New form of scanning optical microscopy," Phys. Rev. B, 39, 767-70 (1989). Reddick et al. discloses transillumination of thin and transparent samples with evanescent light, but the entrance pupil in Reddick et al is not an objective in the conventional microscopy sense. Rather, it is an optical fiber that is scanned over the sample, close to the sample surface. Thus, there is a loss of flux throughput, and vertical resolution is limited by the mechanism that controls the vertical position of the fiber relative to the sample. A means of scanning in the xy plane is also required, preventing true real-time whole-field imaging.
In addition to the use of evanescent fields and scattering for imaging and other purposes, phase shifting interferometry has played important roles in different contexts. For example, the wave nature of light has been beneficially employed in optical microscopy where vertical height resolution is limited to .lambda./2. Here, interference between wavefronts can be employed to increase vertical resolution and contrast. Interference between wavefronts with a static, or fixed phase shift as in differential interference contrast microscopy, invented by Nomarski (G. Nomarski, J. Phy. Rad. 16, 9S (1955)), or phase contrast microscopy invented by Zernike (1935), results in a contrast enhancement so that normally unresolved, substantially subwavelength vertical differences are made visible. Typically, the unifying principle behind the many manifestations of interferometers is that a reference wavefront is made to interfere with an unknown. In the last case, the wavefront is combined with a phase-shifted version of itself.
In other interferometers, a controlled, known reference wavefront is split into two wavefronts. One is disturbed by the sample, and the disturbed wavefront is recombined with the reference. The resulting interference image, or interference map, is analyzed to determine vertical information about the disturbing sample surface.
In all manifestations, phase can be measured to better than one part in one hundred of the wavelength, .lambda.. This high resolution, well beyond the Abbe limit, is termed superresolution, but is only in the vertical axis. Spatial resolution in the XY plane remains at best .lambda./2.
Interference microscopes such as have been available commercially from WYKO and ZYGO also achieve vertical resolution of .lambda./100, or better, through dynamic phase shifting with, for example, a piezo-actuated reference window. The phase shifting causes a multitude of sequential interference images, each the result of a discrete and unique phase shift, in which the interference information is manifested as an amplitude or light intensity variation in the spatial image plane. While the data from each of the multitude of interference images, or maps as they are sometimes called, must be reduced to obtain the final image, the resolution is remarkable and, unlike Nomarski and phase contrast, quantitative. The shifting must be over at least one complete fringe in order to extract full information. The vertical range of this phase shifting technique is about half the wavelength of the illumination.
While the art describes a variety of devices that utilize evanescent field illumination for investigating vertical surface characteristics, there remains a need for improvements that offer advantages and capabilities not found in presently available instruments, and it is a primary object of this invention to provide such improvements.
It is another object of the invention to apply methods and means of phase shifting and phase shifting interferometry to the phase of the inhomogeneous waves comprising evanescent fields to achieve superresolution in the lateral spatial plane.
Another object is to employ lateral phase shifting to achieve superresolution imaging in optical systems using evanescent field illumination, while maintaining whole field (rather than scanning) imaging as in the photon tunneling microscope.
Another object is to control the phase of the evanescent field by illuminating a diffractive structure with a spatial grating period smaller than the illumination wavelength so that the evanescent modes are phase locked to the diffracting structure, and then to provide means to modify the grating period in order to shift the evanescent field phase.
Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.